This invention relates to a composition that inhibits corrosion in metal surfaces, particularly as an additive to paint. The metal surface may be pre-treated with either a chromate or phosphate wash. The invention inhibits corrosion over or on the applied area by forming an oxide layer that is resistant to activities that create thermodynamic instability in metal surfaces, leading to corrosion. The invention is also designed to inhibit corrosion at a level of performance similar to chromate inhibitors; however, the present invention does not incorporate chromium, which is toxic, or any other toxic elements in its composition and is, therefore, non-toxic.
The exposure of metals to water, oxygen, and other elements can lead to deterioration in the strength of the metal. The rusting of steel is an example of such corrosion to metal. Corrosion is an electrochemical process that takes place at the surface of a metal substrate in a gaseous or liquid environment. Interaction with outside elements causes the metal surface to be thermodynamically unstable, thereby becoming an electron and ion conductor and acceptor. One way in which inhibitors function is by reducing the amount of interaction at the surface of the metal by providing ions to react with outside elements and form an oxide layer which serves as a barrier against subsequent outside element reactions.
Inhibitors that deter corrosion in metals have been developed for a number of years. The most effective of these involve the use of chromate based compounds. Chromate compounds function in two ways to prevent the corrosion of a metal surface. Their first function is as an oxidizer which reduces chromium (VI) to chromium (III). Chromium (III) has 3 extra electrons in comparison to chromium (VI) and reacts with the metal ions at the surface of the metal substrate to form a thin barrier film to inhibit the corrosion process. Chromium is very soluble in water and readily liberates its ions as water penetrates the paint film. The chromium ions are carried to the metal/coating interface where they form a thin continuous film on the metal which results in polarization to a passive potential. The rate at which this reaction occurs has a substantial influence on the effectiveness and longevity of inhibition.
The toxicity of chromate based pigments leads to a search for alternatives. At the same time, however, pigments are desired that will deter corrosion with a level of performance similar to that of chromate based pigments. Currently, strontium chromate is the leading inhibitor used to prevent corrosion in such applications as metal deco, coil coatings, and thin film applications. There have been several non-toxic inhibitors, including phosphates, phosphosilicates, ion exchange and organic types that have been tested for anti-corrosion effectiveness. However, these non-toxic inhibitors do not perform as well as chromate compounds. One area in particular where the non-toxic compounds are inadequate is preventing creepage, or corrosive activity, at the cut edge of a metal to which they have been applied. These compounds are not able to duplicate the solubility level of chromium compounds and thus, cannot inhibit corrosion at the same level.
U.S. Pat. No. 5,866,652, to Hager, discloses a chromate-free protective coating. The composition of the inhibitor in the Hager invention requires the ester of a rare earth metal such as cerium and lanthanum oxalates and acetates as well as vanadate salts of alkali and alkali earth metals. Vanadium can cause instability in paint formulas, does not provide efficient solubility, and is toxic. The composition of the present invention does not require any form of vanadium. Thus, the present invention provides excellent stability in paint formulas and only slight drifts in viscosity after 30 days at room temperatures or higher.
Similarly, U.S. Pat. No. 5,064,468, to Okai, describes a corrosion preventive coating composition comprising a specific phosphorus compound, but also comprising a specific vanadium compound. Again, the presence of vanadium in the Okai compound creates a substantial difference from the present invention in that it does not satisfy the main objectives of the present invention, which are inhibition of corrosion, at an effective level of solubility, without toxicity.
Previous versions of chromate-free inhibitors have required carboxylic acids in their formation, i.e., U.S. Pat. No. 4,999,250, to Richardson, and U.S. Pat. No. 5,458,678, to Armstrong. However, carboxylic acids are not used in the creation of the present invention, and are not present in the final composition of the present invention. Furthermore, while carboxylic acids do promote adhesion of a primer to the cut edge, they do not aid the inhibition of corrosion. Coating systems containing the present invention may be applied at a dry film thickness, (xe2x80x9cDFTxe2x80x9d), of 4-8 microns, providing excellent adhesion without the need for carboxylic acids in the composition.
The present invention addresses the need for an effective soluble inhibitor while satisfying the desire for non-toxicity. The composition of the inhibitor of the present invention is based on the following major chemical reactions:
M+CO3+H3PO4xe2x86x92M+HPO4+H2O+CO2 (gas)
CaO+H3BO3xe2x86x92CaHBO3+2H2O
The borate ions provide anodic protection by forming a continuous borate film at the coating/substrate interface, thus providing improved adhesion. In addition, phosphate ions provide solubility at low pH levels. The combination of theses ions provides great synergistic value, in that the present invention inhibits corrosion at the same level as chromium compounds due to its higher percentage solubility.
The preferred embodiments of the composition of the inhibitor are Barium borophosphate and Strontium borophosphate. The present invention allows the metal ions in the inhibitor, preferably barium or strontium ions, to react with outside elements such as nitrites, to form salts. The resulting barrier reduces the amount of corrosion occurring at the surface of the metal. Furthermore, the present invention displays high solubility in a pH range of 2-8, thus allowing for greater liberation of ions than seen by chromium compounds. In addition, by eliminating the need for Chromium in the final composition or in the process of creating the invention, the invention also eliminates toxicity.
The object of the present invention is to inhibit corrosion in a metal surface on or over the applied area at the same performance level of a chromate inhibitor, in particular Strontium chromate. The present invention displays corrosion reduction, when subject to salt spray, similar to strontium chromate. The borophosphate inhibitors of the present invention have displayed similar edge and scribe creepage, in comparison to strontium chromate, on chromate pre-treated hot dipped galvanized steel panels as well as similar overall visual ratings for that system. In a polyester primer system, the inhibitors had less creepage at the cut edge and blistering in the field area for chromate pre-treated hot dipped galvanized steel panels. After applying the same coating to phosphate pre-treated hot dipped galvanized steel, the barium borophosphate composition displayed less creepage at the cut edge and performed better overall in comparison to strontium chromate. Furthermore, the borophosphate inhibitors have displayed excellent solubility in the pH range of 2-8, in comparison to strontium chromate, thus exhibiting a stronger ability to inhibit corrosion.
Another object of the invention is to provide a non-toxic means of inhibiting corrosion of metal. By combining a phosphorus compound, comprised of an acid and a metal selected from the group consisting of an alkali, alkaline earth, or transition metal, and a boron compound, comprised of an acid and a metal selected from the group consisting of an alkali, alkaline earth, or transition metal, the resulting composition provides corrosion inhibition comparable to strontium chromate without comparable toxicity.
Numerous other advantages and features of the invention will become readily apparent from the detailed description of the preferred embodiment of the invention and from the claims.
While the invention is susceptible of embodiment in many different forms, there will be described herein in detail a preferred embodiment of the invention. It should be understood, however, that the present disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the spirit and scope of the invention and/or claims of the embodiment illustrated.
The present invention is a corrosion preventive pigment comprising a phosphorus compound, from an acid, and an alkali, alkaline earth, or transition metal; and also comprising a boron compound, from an acid, and an alkali, alkaline earth or transition metal.
The phosphorus compound is prepared by reacting a metal carbonate with phosphoric acid, preferably. The result is a metal phosphate, water and carbon dioxide gas. The boron compound is prepared by reaction of calcium oxide and boric acid, preferably, to yield calcium borate and water. The reactions are as follows:
M+CO3+H3PO4xe2x86x92M+HPO4+H2O+CO2 (gas)
CaO +H3BO3xe2x86x92CaHBO3+2H2O
These reactions are performed in hot water inside of a batch mixer. The wet slurry is dried and the particle size is reduced for incorporation into paint. The inhibitor is then used in a primer coat that is applied at a dry film thickness of 4-8 microns while the top coat may be applied at a dry film thickness of 28-55 microns. The inhibitor then provides optimum corrosion protection for environments with a pH range of 2-8.
Without intending to limit the scope of the present invention, the method of creating the composition may be better understood by referring to the following examples: